1. Field of the Invention
This invention relates to electrolytic cell monitoring for electrometallurgical systems, including, for example, i) electrorefining and electrowinning systems for copper, zinc, nickel, lead, cobalt, and other like metals, ii) electrochemical cells, such as chlor-alkali systems, and iii) molten salt electrolysis, such as aluminum and magnesium electrolysis.
Insofar as the inventive arrangements can be used with electrolytic cell monitoring during a copper refinement stage of producing copper, copper production is described for exemplary, representative, and non-limiting purposes.
2. Description of Related Art
Producing copper involves a series of steps involving mining, crushing and grinding, concentrating, smelting, converting, and refining procedures, each of which is well-known, shown in block diagram format in FIG. 1. As depicted, mining 10 loosens and collects ore. Crushing and grinding 12 turn the ore into a crushed and ground ore, comprising a fine powder in which desired ore minerals are liberated. Concentration 14 collects the desired ore minerals into a watery slurry, which is then filtered and dried to produce a liquid concentrate suitable for smelting. Smelting 16 smelts (i.e., melts and oxidizes) iron and sulfur in the liquid concentrate to produce a copper matte. Conversion 18 converts the copper matte by oxidation into a blister copper and finally, refinement 20 refines the blister copper into a finished product, i.e., cathode copper, typically with a copper purity of 99.99% or better.
The blister copper is refined, usually first pyrometallurgically and then electrolytically. More specifically, the blister copper is subjected to an additional purification step to further up-grade the copper content, such as fire refining in a reverberatory or rotary anode furnace. Then, the blister copper is cast into large, thick plates called anodes, which are often transferred from an anode casting plant to the electrolytic copper refinery by truck, rail, or the like.
In the electrolytic copper refinery, the anodes are lowered into an acidic solution that contains approximately 120-250 gpl of free sulfuric acid and approximately 30-50 gpl of dissolved copper. The anodes are also electrically connected to a positive direct current supply. To electrolyze the anodes in this aqueous electrolyte, they are separated by insoluble, interleaved stainless steel blanks called starter sheets or cathodes, which are negatively charged. Electricity is then sent between the anodes and cathodes for a pre-determined length of time, causing copper ions to migrate from the anodes to the cathodes to form plates at the cathodes, which contain less than 20 parts per million impurities (i.e., sulfur plus non-copper metals, but not including oxygen). Voltages of approximately 0.1-0.5 volts are generally sufficient to dissolve the anodes and deposit the copper on the cathodes, with corresponding current densities of approximately 160-380 amps/m2. With each anode producing two cathode plates at which the refined copper is deposited, the cathode plates are then washed and ready for an ultimate end use.
In a typical copper refinery producing 300,000 tons of copper cathode per year, there can be as many as 1,440 electrolytic cells, each with 46 anodes and 45 cathode blanks, for a total of 131,000 pieces suspended into the cells. In such a traditional copper refinery, each cathode and each anode is electrically connected to the refinery current supply system through two or more contact points on the supporting ears of the anodes and the hanger bars of the cathodes. This means there can be a total of over 260,000 electrical connections (i.e., two per anode and two per cathode multiplied by the number of cathodes and anodes).
Critical to the efficient operation of the refining process is the absence of open and short circuits between the anodes and cathodes. As subsequently elaborated upon, short circuits can occur if the anodes and cathodes are mis-aligned or if copper deposits on the cathode grow in a non-uniform manner and contact the anode. When short circuits do occur, the desired copper plating process is disrupted. Open circuits, on the other hand, can occur if there is poor contact between the current supply and the anodes or cathodes. When open circuits do occur, the efficiency of electrical use decreases.
Thus, refinement 20 refines the blister copper into refined copper, which typically contains approximately 99.99 wt % copper (i.e., effectively, pure copper). Thereafter, refinement 20 allows the refined cathode copper to be converted into any number of copper end-products using conventional methods and techniques, which are well-known in the art.
The efficiency of copper refinement 20 can be increased by increasing the efficiency of cell monitoring. More specifically, at least two important cell parameters need to be closely monitored—namely, cell voltage and cell temperature. Failure to adequately monitor these two cell parameters, and others, can reduce metal recovery, increase scrap rate, and lead to inefficient energy utilization. Nevertheless, most electrolytic metal recovery and refining facilities do not effectively monitor these cell parameters, primarily due to high capital and operating costs associated with such cell monitoring. For example, these costs are significantly high when each individual electrolytic cell in a tank house is hardwired to parameter monitoring and transmission equipment. Doing so generally requires a significant amount of hardwiring in an environment that is inherently hostile, inherently corrosive, and inherently subject to large magnetic fields. In particular, while the voltage differential across any cell is on the order of 0.1 to 0.5 volts, the voltage differential across the entire tank house can be several hundred volts. It is inherently unsafe to simply connect wires to the individual cells and route these to voltage monitoring equipment because the voltage potential can be potentially fatal. Because presently existing cell monitoring equipment and technologies are expensive and require extensive hard wiring, both shortcomings have significantly deterred widespread market penetration of effective electrolytic cell monitoring.
As a result, open and short circuits commonly occur during the electrolytic refining of copper. They occur for many reasons, including i) poor anode and cathode physical qualities, ii) poor contact between the current supply and the anodes or cathodes, iii) misalignment of anodes and cathodes, and iv) localized variations in electrolyte temperature, additive levels, or chemistry. Thus, efficient electrolytic cell monitoring is important during the electrolytic refining of copper, as it can enable system operators to detect open and short circuits between anodes and cathodes, which, if not cleared, reduce current efficiencies and result in down-stream processing problems, such as poor cathode development. As known, copper impurity, copper content, and copper appearance are also ultimately adversely affected by open and short circuits.
Conventional monitoring focuses on only identifying short circuits between the anodes and cathodes. This is commonly accomplished by manually using a hand-held Gauss meter to detect abnormal magnetic fields flowing through the cathode. Such a procedure generally requires physically walking over the anodes and cathodes in each cell while closely observing the hand-held Gauss meter to detect a large deflection in a meter needle. Oftentimes, the Gauss meter is affixed to a distal end of a long stick or pole by which it can then be held close to the cathode hanger bar. Regardless, the task is both ergonomically difficult and accident-prone. Moreover, walking on the cells frequently misaligns the anode and cathodes, can lead to possible contamination, and often leads to further problems as well.
While detecting open and short circuits deals with their effects rather than their causes, it is a widely recognized technique for improving electrode quality. Accordingly, after a short circuit is detected, it is generally cleared by probing between the cathode and anode with a stainless steel rod to locate the fault and then physically separating (i.e., breaking off) an errant copper nodule growing at the epicenter of the short circuit. This often requires physically lifting the cathode out of the cell. Unfortunately, however, many open and short circuits are not often detected until after significant damage has already occurred.
Consequently, there is a need for less expensive, less intrusive, lower maintenance, and higher efficiency electrolytic cell monitoring systems and methods. Such systems and methods would increase energy utilization and efficiency during the copper refinement stage 20 of producing copper. Thus, a need exists for a cost effective, minimally intrusive, minimal maintenance, and increasingly efficient electrolytic cell monitoring systems and methods for measuring electrolytic cell parameters such as anode and cathode voltages and temperatures during the copper refinement stage 20 of producing copper.